Nice introduction. I missed two things that may not be clear for beginners.
The image shows 5 containers (or container like objects) but there are only 4. The app is included via volumes. That confused me when i saw the pic but i got it when i read the article.
I think you forgot to explain the "links" statement as part of the web container. With that it's possible to directly address the app container in the nginx config.
Nice introduction. I missed two things that may not be clear for beginners.
Keep going! :)
Thank you for the comment. I'll update it shortly.